The present disclosure relates to compact mercury-free high intensity discharge (HID) lamps and particularly compact HID lamps intended for automotive headlamp applications. Heretofore, high intensity discharge lamps for general lighting, and likewise, HID lamps for automotive headlamp applications have employed mercury for providing a desired lamp voltage and luminous flux of the lamp, as well as spatial luminance distribution of the emitting discharge arc volume and spectral power density distribution of the emitted light. In case of automotive applications, adequate spatial distribution of arc luminance (in a simplified approach, arc width, arc bending and arc length) is one of the most important characteristics for providing the required forward road illumination from the headlamp of a car during nighttime vehicle operation.
However, it has been desired to eliminate the use of mercury in the manufacture of such discharge lamps to minimize the risks of exposure to toxic materials during manufacturing and to prevent unwanted distribution of these toxic materials during disposal of these lamps upon removal from service or in case of accidental spill of toxic materials in the event of an accident.
Heretofore, there have been several attempts to replace mercury in high intensity discharge lamps. The basic initial and through life optical characteristics of contemporary mercury-free HID lamps are approaching that of their mercury containing counterparts'; however, not all of these technical requirements can fully be satisfied. As an example, mercury-free high intensity discharge lamps have been found to exhibit a discharge arc having a thinner and more curved arc shape geometry than the arc present in mercury-filled high intensity discharge lamps.
In addition, the profile of the arc luminance distribution of a mercury-free discharge arc along the length of the arc between the spaced and opposing electrodes in the discharge chamber still contains “focal spots” or localized regions of higher intensity in the region adjacent the electrodes. An example for the axial arc centerline temperature distribution of a mercury-free high intensity discharge arc is shown in FIG. 5; wherein it is seen that the central region of the arc reaches temperatures of about 5000-6000 K, whereas the regions at the ends of the arc, i.e. adjacent the electrodes, it spikes to reach temperatures of 7000-8000 K.
It has thus been desired to provide a way or means of leveling out both the lateral and axial spatial luminance distribution of the discharge arc in a mercury-free discharge lamp. A “homogenized” arc luminance distribution from a such mercury-free HID lamp would significantly simplify the optical system required to distribute the light rays from the arc into a focused beam, either by the use of mirror reflectors or lenses.
Referring to FIG. 1, the area of desired illumination of the road surface for automotive headlamp applications is shown for “low beam” or “passing beam” illumination mode. This type of road illumination has a sharp light-to-dark transition, or “cut-off line” along a horizontal line in the direction of the opposing lane (line A) and another “cut-off line” along a line tilted upwards by an amount of about 15° in the direction of the travelling vehicle lane (line B). Referring to FIG. 2, the typical illumination levels of the mercury-free arc discharge headlamps from the travelling vehicle is shown for “low beam” mode headlamp operation with the darkest region indicating the area of greatest illuminance. FIG. 3 shows the typical distribution of illumination for a desired “high beam” or “driving beam” headlamp operation mode of a travelling vehicle with the greatest illuminance shown in the darkest shading.
As mentioned above, it has been desired to compensate for the increased curvature and thinness of a mercury-free discharge arc in order to provide a desired arc image spatial luminance distribution constituted by the optical rays emanating from the arc in a manner which is easy to incorporate into the manufacture of the high intensity arc discharge lamp without significantly increasing the cost of the lamp.